Hewlett-Packard accuses the UK software group it bought of overvaluing itself after it loses £5.5bn on the deal

US tech giant Hewlett-Packard has accused former staff of UK software firm Autonomy of a 'wilful effort' to mislead it when it bought the company last year - an allegation denied by its ex-boss.

HP wrote off $8.8billion (£5.5billion) in its fourth quarter earnings report, and linked the majority to 'serious accounting improprieties, misrepresentation and disclosure failures' discovered by an internal investigation.

It acquired Cambridge-based Autonomy in a £7.1billion deal that saw its founder Dr Mike Lynch make around £500million. In a statement to Reuters, a spokeswoman for Lynch said the former management team of Autonomy was 'shocked' by HP's statement and 'flatly rejects these allegations, which are false'.

Tech giant: Hewlett-Packard, based in Palo Alto in California, has taken a £5.5billion hit after accusing staff at Autonomy - a British company it purchased last year - of cooking the books to increase its company value

Shares in HP, which are traded on the Nasdaq, tumbled 13 per cent or $1.72 to £11.58 in early trading in New York.

The American firm, which alleges that former members of Autonomy's management changed accounts in a bid to 'mislead investors and potential buyers', has referred the matter to the Serious Fraud Office and US authorities.

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Autonomy was considered one of the UK's biggest technology success stories, having pioneered software that helps companies to search data, particularly in cases involving compliance and legal issues. Clients include Procter & Gamble, Nestle and Shell.

HP's internal inquiry, run by accountancy firm PwC, was launched after a whistleblower at Autonomy approached it with allegations.

In a statement, HP said it was 'extremely disappointed' by the findings of the investigation.

It added that the actions of former staff 'appear to have been a wilful effort to mislead investors and potential buyers, and severely impacted HP management's ability to fairly value Autonomy at the time of the deal'.

But it went on: 'We remain 100 per cent committed to Autonomy and its industry-leading technology.'

HP said more than $5billion (£3.1 billion) of the impairment charge announced today was linked to the allegations, while the remainder was connected to the recent trading value of HP stock.

After the PwC investigation, HP alleges that Autonomy was 'substantially overvalued' at the time of its acquisition due to a mis-statement of Autonomy's financial performance.

As well as the SFO in the UK, HP has referred the matter to the US Securities and Exchange Commission's Enforcement Division and is preparing to seek compensation against various parties.

The impairment charge has pushed HP to a $6.9billion (£4.3billion) loss in the fourth quarter, compared with a $200million (£125million) profit in the same quarter last year.